The present invention relates to improvements in the coating of substrates of a glassy siliceous material with thin films of a metal or metal-alloy and more particularly to the coating of sheets of glass by the known sputtering or sputter-coating process.
The process of sputter-coating involves ion bombarding a target of the coating material in low-pressure gaseous glow discharge to cause atomic particles of coating material to be dislodged and deposited on the substrate to be coated. One type of apparatus employed for carrying out the process embodies a plurality of aligned, independently controlled contiguous chambers including an entry chamber, a coating chamber and an exit chamber, sputter-coating means in the form of a substantially rectangular cathode mounted horizontally in the upper portion of the coating chamber and having a sheet of a selected coating material secured to the bottom thereof, and conveyor means for moving the substrates (glass sheets) in a horizontally disposed position through the successive chambers and beneath the sputter-coating means such that a continuous film of the selected coating material will be deposited on the substrate.
In one such process, the glass sheets or other substrates to be coated are placed horizontally directly upon a solid metal platen which supports and conveys the sheets through the successive chambers of the coating apparatus. While this method has proven commercially acceptable, the use of metal platens for supporting the glass sheets has sometimes been attendant with certain objections. Although the specific reasons are not fully understood, it is believed that the metal of which the platens is made has a tendency toward outgassing and that these gases comingle with the coating material being deposited resulting in an oxide coating rather than a coating of pure metal. This has undesirable effects on the deposited coating, particularly at the edges or marginal portions of the glass sheet in that the color, as well as the thickness and composition of the coating are adversely affected.
According to the present invention, these objections have been effectively overcome by providing the metal platen with a covering or overlay which serves as a support bed for the glass sheets during the coating operations. It has been found that significant improvement can be obtained in the thickness and composition of the coating, as well as the color of the glass sheet at the edges thereof by overlaying the upper surface of he metal platen with one or more plates of glass and then laying the glass sheets to be coated on the glass overlay and out of direct contact with the metal platen.
The primary object of the invention, therefore, is to provide novel means for supporting the glass sheets during coating such that the sheets will be uniformily coated from edge to edge with a thin film of metal of uniform thickness, composition and color.